CA2784644A1

CA2784644A1 - Preform for producing plastic containers in a two-stage stretch blow-molding process

Info

Publication number: CA2784644A1
Application number: CA2784644A
Authority: CA
Inventors: Robert Siegl
Original assignee: Alpla Werke Alwin Lehner GmbH and Co KG
Current assignee: Alpla Werke Alwin Lehner GmbH and Co KG
Priority date: 2009-12-23
Filing date: 2010-10-02
Publication date: 2011-06-30
Also published as: PT2516121E; CN102695591B; US20120231191A1; RU2012131428A; US8617676B2; BR112012013893B8; CH702464A1; UA106407C2; BR112012013893B1; CN102695591A; HRP20140490T1; PL2516121T3; EP2516121A1; BR112012013893A2; MX2012006085A; RU2540301C2; ES2461965T3; EP2516121B1; WO2011076301A1

Abstract

The invention relates to a preform (1) for producing plastic containers in a two-stage stretch blow-moulding process, wherein the preform has an elongated preform body (2), the one longitudinal end of which is closed by a bottom (3), while the other longitudinal end adjoins a neck section (4) having threaded sections (5) or similar positive protrusions. The preform (1) is produced from a plastic material suited for the stretch blow-moulding process, said material having a refractive index of 1.3 to 1.6 at a temperature of 100°C to 120°C. The preform bottom (3) is designed such that the outer wall (18) thereof and the inner wall (17) thereof delimit a flat divergent lens. The outer wall (18) and the inner wall (17) of the preform bottom (3) designed as a divergent lens have radii of curvature (c, b) which are greater at least by a factor of 1.4 than the radii of curvature (s, r) of the associated outer wall (8) or inner wall (7) in the region of the preform body.

Description

Preform for Producing Plastic Containers in a Two-Stage Stretch Blow-Molding Process The invention relates to a preform for producing plastic containers in a two-stage stretch blow-molding process according to the preamble of Claim 1.

A large number of plastic containers that are now used - in particular, for example, plastic flasks and the like - are produced in a stretch blow-molding process.
In this method, a so-called preform - which in most cases has an elongated, tube-like shape and has a base on its one longitudinal end and a neck region with formed threaded sections or the like on the other longitudinal end - is inserted into a mold cavity of a blow mold and blown in through a medium that is injected with overpressure. In this case, the preform is additionally elongated in the axial direction with an elongated mandrel that is inserted through the neck opening. After the elongation/blow-molding process, the finished plastic container is demolded from the blow mold.

The single- or multi-layer preform is usually produced in a separate injection-molding process before the stretch blow-molding process. It has also already been proposed to produce preforms in a plastic extrusion press method. As raw material for the production of plastic containers in the stretch blow-molding process, primarily polypropylene or PET
(polyethylene terephthalate) is used. Polypropylene and PET have been tested multiple times, and their properties are adequately known. In the so-called single-stage stretch blow-molding process, the preform is inflated and elongated directly after its production to form a plastic container. In many cases, however, the plastic containers are produced in a two-stage method at a different place and time from the stretch blow-molding process and are intermediately stored for later use. In the later stretch blow-molding process, the preforms are heated again, introduced into a blow mold, stretched with an elongated mandrel in the longitudinal direction, and inflated according to the mold cavity by overpressure to form a plastic container. In this way, both processes - the injection-molding and the stretch blow-molding - can be operated separately and optimally.

The preforms that are used in the stretch blow-molding method usually have an elongated shape and have a convex base that is curved outward. The neck region of the preform is already completely formed and is provided with threaded sections or similar positive protrusions, which make it possible to attach a closure or cover, which is equipped with correspondingly designed locking elements. In the two-stage process, the preforms have to be heated again to their deformation temperature range before the stretch blow-molding. To this end, the preforms are plugged with their neck regions into finger-like holding devices and transported through a heating station. In many cases, the heating of the preforms is carried out via infrared radiation or near-infrared radiation, which is generated by quartz tube radiators. For better use of the energy emitted by the quartz tube radiators, one or more mirrors are provided that reflect the electromagnetic radiation.
In the heating station, the preforms are transported between the quartz tube radiators and the mirrors facing them. Through the bomb-shaped base of the preform, the irradiated infrared radiation moves by scattering or directly even to the finger-like holding devices of the preforms, which are thus heated. To thus prevent deformations of the neck region of the preform that is formed with high accuracy, the finger-like holding devices have to be cooled. Since the absorbed irradiated energy often cannot be drained off to a sufficient extent even by the cooling of the holding devices, the preform neck often has to be designed with a larger wall thickness than would be necessary for the plastic container that is to be manufactured from the preform.
During stretch blow-molding, the preform is stretched longitudinally using an elongated mandrel. In the region of the support surface of the base of the preform with the elongated mandrel, the base cools relatively quickly, and an undesirable accumulation of amorphous material can occur in the base region of the plastic container that is produced in the stretch blow-molding process.

The object of this invention is therefore to remedy these drawbacks of the preforms of the state of the art. A preform is to be provided that makes it possible to design the neck region also with reduced wall thicknesses. In the further processing of the preform to form a plastic container in the two-stage stretch blow-molding process, undesirable accumulations of amorphous material in the base region of the container are to be avoided.

These and even still further objects are achieved according to the invention by a preform with the features that are listed in Claim 1. Further developments as well as advantageous and preferred variant embodiments of the invention are the subjects of the dependent claims.

A preform for producing plastic containers in a two-stage stretch blow-molding process is proposed by the invention, and said preform has an elongated preform body whose one longitudinal end is sealed with a base and to whose other longitudinal end a neck section with threaded sections or similar positive protrusions is connected.
The preform is manufactured from a plastic that is suitable for the stretch blow-molding process, which has a refractive index of 1.3 to 1.6 at a temperature of 10 C to 120 C. The preform base is designed in such a way that its outside wall and its inside wall delimit a flat divergent lens.
The outside wall and the inside wall of the preform base that is designed as a divergent lens in this case have radii of curvature that are larger by at least the factor 1.4 than the radii of curvature of the related outside wall or inside wall in the region of the preform body.

In combination with the refractive index of the preform material, configuring the preform base as a flat divergent lens causes the irradiated electromagnetic heat radiation to be deflected away from the finger-like holding device. By the configuration of the preform body according to the invention, it is undertaken to absorb a larger proportion of the introduced electromagnetic heat radiation in the preform base and in the preform wall. As a result, less radiation energy reaches the finger-like holding device of the preform during its transport through the heating station, and the holding device is considerably less heated. The neck portion of the preform, which is in direct contact with the finger-like holding device, is thus also less heated. As a result, the risk of a deformation of the neck portion is considerably reduced, and there is the possibility of designing the neck portion with a smaller wall thickness. The decrease in the wall thickness of the preform in the neck portion leads to a reduction of expensive raw material. Specifically in mass-produced articles such as plastic containers, a material reduction has economic or else ecological advantages.

Overall, the design according to the invention results in a flattening of the preform base. As a result, during the stretching process, there is at first only a small region to form a contact between the elongated mandrel, whose front end has a small radius of curvature, and the preform base with a comparatively large radius of curvature. Only at very high elongation speeds and pressures and toward the end of the mechanical elongation process is this contact region increased. As a result, the local cooling of the perform base is limited to a very small region, and undesirable accumulations of amorphous material in the base region of the plastic container that is produced can be avoided. Rather, the as-yet not cooled plastic material in the preform base is available for the rest of the blow-molding process. This also makes possible a reduction of material in the base of the preform.

In one variant embodiment of the invention, the preform base that is designed as a flat divergent lens in the region of the axis of the preform or in the center of the divergent lens has a wall thickness that is at least 0.2 mm smaller than a wall thickness of the preform base at the transition in the preform body.

The base of the preform is configured in particular in such a way that an electromagnetic heat radiation of a wavelength of 0.5 m to 2 m, which is introduced into the region of the base essentially perpendicular to the preform axis, is absorbed to a significant extent by total reflection within the base and/or the body of the preform. It is thus ensured that very little electromagnetic radiation reaches the finger-like holding device, and the neck portion that is in contact with the holding device is less heavily heated. By a larger proportion of the introduced heat radiation being absorbed in the preform base and/or the preform body, the efficiency of the preform heating is also increased.

The preform base can be designed plano-concave or convex-concave. In this case, the terms "plane" or "convex" relate to the first surface on which the electromagnetic radiation takes place, i.e., to the outside wall of the preform base. The term "concave"

relates to the opposing inside wall of the preform base. The outside wall of the base of the preform is to have a larger radius of curvature than the inside wall of the preform in the region of its base. In the case of a flat design of the outside wall, the radius of curvature is infinitely large.

Preforms, which are designed according to the invention and are provided for further processing in a two-stage stretch blow-molding process, advantageously consist of plastics or plastic mixtures from the group that consists of polyester, PET
(polyethylene terephthalate), polyolefins, polystyrenes, and PLA (polylactic acids).

The preform according to the invention can be composed of one or more layers depending on the application provided. It can also comprise barrier additives, in particular oxygen traps, nanoclays or UV blockers. In another variant embodiment of the invention, the preform that is composed of multiple layers can also have a barrier layer against oxygen and/or UV radiation and/or a slide coating and/or a residual discard coating.

The preform according to the invention is produced, for example, in a plastic injection method. Plastic injection methods or injection-molding methods have been tested sufficiently and result in preforms with the desired accuracy. In this case, the feed point of the preform is suitably located in the region of the base. In the plastic container that is produced from the preform, it is thus generally not visible in the deployed position.

The plastic extrusion press method represents an alternate production method for the preform, which also leads to high-quality results and is very well suited for mass production.
The preform that is designed according to the invention can also be produced in an extrusion blow-molding method. This production method that has been recently used to an increased extent is distinguished by its high throughput and low production costs and is also suitable in particular for preforms that are composed of multiple layers.
Multi-layer preforms can also be produced in a so-called "overmolding" method.

The preform that is designed according to the invention can be provided at least in places with a color that deviates from the usual preform body or can have at least one color layer in a multi-layer variant embodiment. The varying coloration or the color layer can also be used, i.a., to absorb - even better and specifically in the preform material - the radiation energy that is introduced when the preform is heated.

In another variant embodiment of the preform, it can also be provided that the latter has an outside wall in its base region that has a greater roughness than an outside wall of the body of the preform. The increased roughness can also be used for a better absorption of the introduced radiation energy in the preform material.

A variant embodiment of the preform that is advantageous relative to the reduced use of material has a neck portion that has - in the region of the threaded sections or similar positive protrusions - a minimum wall thickness that is smaller by at least 20% than a mean wall thickness in the region of the preform body.

In another variant embodiment of the invention, the neck portion in the region of the threaded sections or similar positive protrusions, in particular on the threaded base, has a minimum wall thickness that is smaller than 1.34 mm.

Plastic containers, which are manufactured in a two-stage stretch blow-molding process from a preform that is designed according to the invention, in many cases have a better and more homogeneous material distribution than conventional plastic containers of the state of the art, and thus have more uniform properties of strength relative to mechanical and thermal stresses, for example in applications in which the contents are dispensed hot.

Further advantages and variant embodiments of the invention follow from the description below of an embodiment with reference to the diagrammatic drawings. Here, in depictions that are not to scale:

Fig. 1 shows a preform according to the invention in an axial section on half a side in a heating station; and Fig. 2 shows a preform according to the invention in an axial section on half a side.

Fig. 1 diagrammatically shows a preform with half a side axially cut away, which is provided overall with the reference number 1 during its transport through a heating station 30. The preform 1 has an elongated preform body 2, whose one longitudinal end is sealed with a preform base 3. A neck portion 4, on whose outside threaded sections 5 or the like are made, is connected to the opposing end section of the preform body 2. The threaded sections 5 or the like allow the screwing-on of a closure or cover that is equipped with corresponding locking elements. The preform 1 is produced, for example, in a plastic injection method or in an extrusion press method. It can also be produced in an extrusion blow-molding method. The preform 1 is an intermediate product of the two-stage stretch blow-molding process in which first the preform 1 is produced and, at a different time and place, the preform is reshaped by axial stretching and radial inflation to form a plastic container. The two-stage stretch blow-molding process has the advantage that the preform production and the production of the plastic container can be carried out independently of one another in each case with the optimum clock rate.

So that the preform 1 can be stretched in the stretch blow-molding device and inflated by overpressure, it first must be heated again to a temperature that is necessary for the stretch blow-molding process. To this end, it is transported through one or more heating stations 30. The heating station 30 comprises a number of heat lamps, usually quartz tube radiators 31, which emit electromagnetic radiation R in the near-infrared and infrared ranges. The wavelength of the emitted radiation is in the range of 0.5 m to 2 FLm.
Usually, several quartz tube radiators 31 are arranged one on top of the other. A reflector arrangement 32, for example metal reflectors, is provided facing the quartz tube radiators 31, which reflects the electromagnetic radiation R that is emitted by the quartz tube radiators.

The preform 1 is transported through a channel between the quartz tube radiators 31 and the reflector arrangement 32. To this end, it is plugged in headfirst with a neck portion 4 on a finger-like holding device 35, which is transported continuously or clocked through the heating station 30. Usually, in this case, the finger-like holding device 35 is also still rotated around its axis, so that the preform 1 is heated from all sides. The finger-like holding device 35 moves below a stationary or movable partition 33, which is provided with a slot-shaped opening 34 for the preform 1. The partition 33 is to prevent the heating electromagnetic radiation R from the quartz tube radiators 31 or from the mirror arrangement 32 from moving to the finger-like holding device 35 and the neck portion 4 of the preform 1. In most cases, the finger-like holding device 35 is provided in addition with a cooling, for example a water cooling, to prevent it from being heated excessively. Because of this heating, the neck portion 4 of the preform 1 that is manufactured with high precision and that is in indirect contact with the finger-like holding device 35 could otherwise soften and become deformed.

Because of the partition 33, relatively little electromagnetic radiation reaches the finger-like holding device 35. The highly bomb-shaped preform base represents a problem, however, in the preforms of the state of the art. This leads to the fact that electromagnetic heat radiation that is introduced into the region of the base moves by diffraction and multiple reflections to the finger-like holding device 35 and heats the latter. To remedy this problem, the preform 1 according to the invention is manufactured from a plastic that is suitable for the stretch blow-molding process and that has a refractive index of 1.3 to 1.6 at a temperature of 10 C to 120 C. The preform base 3 has an inside wall 17 and an outside wall 18 that define a flat divergent lens. To this end, the radii of curvature r, s of the inside wall 10 17 or the outside wall 18 of the preform base (3) have radii of curvature b, c that are larger at least by a factor of 1.4 than the related radii of curvature r, s of the inside wall 7 and the outside wall 8 of the preform body 2. In the region of axis A of the preform 1, which at the same time forms the center of the divergent lens, the preform base 3 has a wall thickness that is at least 0.2 mm smaller than in the region of the transfer to the preform body 2. In particular, the preform base 3 is designed in such a way that electromagnetic radiation, which is introduced essentially perpendicular to the preform axis A, of a wavelength of 0.5 m to 2 .tm is absorbed to a significant extent by total reflection within the preform base 3 and/or the body 2 of the preform 1.

Fig. 2 shows the preform according to the invention which again is referred to overall with the reference number 1 in a depiction that is axially cut on half a side.
The elongated, usually cylindrical preform body carries the reference number 2, and the preform base is referred to as 3. The neck portion that is adjacent to the preform body 2 carries the reference number 4, and the threaded sections are indicated at 5. The axis of the preform is provided with the reference number A. A transfer ring 6 separates the neck portion 4 from the preform body 2. The transfer ring 6 is used to transport and to support the preform and the plastic container produced therefrom in certain unit sections of the stretch blow-molding device. In the region of the preform body 2, the preform I has an inside wall 7 as well as an outside wall 8. The inside wall 7 in the region of the preform body 2 has a radius of curvature r. The outside wall 8 of the preform body 2 has a radius of curvature that is provided with the reference number s. In the region of the preform base 3, the inside wall is provided with the reference number 17, and the outside wall is provided with the reference number 18. The inside wall 17 in the region of the preform base 3 has a radius of curvature that is indicated at b, and the outside wall 18 in the region of the preform base 3 has a radius of curvature that is indicated at c.

The preform base 3 is designed according to the type of a piano-concave or convex-concave divergent lens. The terms "piano" or "convex" refer in this case to the first surface on which the irradiated electromagnetic radiation strikes, i.e., on the outside wall 18 of the preform base 3. The term "concave" relates to the opposing inside wall 17 of the preform base 3. The outside wall 18 of the preform base 3 has a larger radius of curvature c than the inside wall 17 of the preform body. In the case of a flat design of the outside wall 18 of the preform base 3, the radius of curvature c is infinitely large.

Because of the design of the preform base 3 according to the invention, the bulk of the electromagnetic radiation, irradiated in the region of the preform base 3, of the wavelength 0.5 m to 2 m is absorbed by total reflection within the preform base 3 and/or the body 2 of the preform 1 or is reflected outward again from the inside wall of the preform 1. Total reflection takes place with the transition from the optically denser medium to the optically thinner medium. An electromagnetic radiation that is transmitted from an optically denser medium (medium with a higher refractive index nl) into an optically thinner medium (medium with a smaller refractive index n2) is broken away according to the Snellius refraction law at the interface of the axis of incidence. The refractive angle is larger than the angle of incidence of the electromagnetic radiation (e.g., infrared radiation). If the angle of incidence is increased, the refracted beam, starting at a certain angle, runs parallel to the interface. This critical angle is also the angle of total reflection. The angle of total reflection comes out as arcsine (n2/nl). Owing to the design of the preform base 3 according to the invention, the bulk of the electromagnetic radiation that is irradiated flat in the base region is absorbed.

The flat design of the preform base 3 also has advantages with respect to the interaction of the preform base 3 with the elongated mandrel. During stretch blow-molding, the perform 1 is elongated using an elongated mandrel in longitudinal direction. The elongated mandrel has a relatively small radius of curvature on its free front end, while the radius of curvature b of the inside wall 17 of the preform base 3 is relatively large. Thus, during the stretching process, it results only in a very small contact area between the preform base 3 and the elongated mandrel. As a result, the preform base 3 cools to a lesser extent, and the plastic material that is found therein is further available for the stretching and blow-molding process.

The preform 1 that is designed according to the invention for further processing in a two-stage stretch blow-molding process consists of plastics that are suitable for the stretch blow-molding process, which at a temperature of 10 C to 120 C have a refractive index of 1.3 to 1.6, for example polyester, PET (polyethylene terephthalate), polyolefins, polystyrenes, and PLA (polylactic acids) or mixtures thereof. The preform 1 can be composed of one or multiple layers. It can be provided with additives that are used as barriers against oxygen, water vapor, or carbon dioxide and/or with fillers.

The preform 1 can have one or more color layers and/or barrier coatings and/or slide coatings and/or residual discard coatings.

Because of the poor heating of the finger-like holding device during transport through the heating station, the neck portion 4 of the preform 1 is also heated to a lesser extent. As a result, it can be designed in the neck portion with a smaller wall thickness than conventional preforms with heavy bomb-shaped bases. Thus, preforms can be used whose neck portions in the region of threaded sections or similar positive protrusions have a minimum wall thickness w that is smaller by at least 20% than a mean wall thickness in the region of the preform body.

Claims

1. Preform for producing plastic containers in a two-stage stretch blow-molding process, with an elongated preform body (2), whose one longitudinal end is sealed with a preform base (3) and to whose other longitudinal end a neck portion (4) with threaded sections (5) or similar positive protrusions is connected, characterized in that the preform (1) is manufactured from a plastic that is suitable for the stretch blow-molding process, which has a refractive index of 1.3 to 1.6 at a temperature of 10°C to 120 C, and in that an outside wall (18) and an inside wall (17) of the preform base (3) delimit a flat divergent lens, and in each case have radii of curvature (c, b) that are larger by at least a factor 1.4 than a related radius of curvature (s) of an outside wall (8) or a radius of curvature (R) of an inside wall (7) of the preform (1) in the region of the preform body (2).

2. Preform according to Claim 1, wherein the preform base (3) that is designed as a flat divergent lens has a wall thickness, in the region of the preform axis (A), that is smaller by at least 0.2 mm than its wall thickness in the region of its transition into the preform body (2).

3. Preform according to one of the preceding claims, wherein its preform base (3) is designed as a flat divergent lens in such a way that an electromagnetic radiation (R) of a wavelength of 0.5 µm to 2 µm, which is introduced essentially perpendicular (A) to the preform axis, is absorbed to a significant extent by total reflection into the preform base (3) and/or into the preform body (2).

4. Preform according to one of the preceding claims, wherein it is manufactured from a plastic or from a plastic mixture of the group that consists of polyester, PET, polyolefins, polystyrenes and PLA.

5. Preform according to one of the preceding claims, wherein it comprises one or multiple layers.

6. Preform according to one of the preceding claims, wherein it is produced in a plastic injection method, and the injection point is located in the region of the base (3) of the preform.

7. Preform according to one of Claims 1-5, wherein it is produced in a plastic extrusion press method.

8. Preform according to one of Claims 1-5, wherein it is produced in an extrusion blow-molding method.

9. Preform according to one of the preceding claims, wherein it is composed of multiple layers and has at least one color layer.

10. Preform according to one of the preceding claims, wherein in its base region, it has an outside wall that has a greater roughness than an outside wall of the body of the preform.

11. Preform according to one of the preceding claims, wherein in the region of the threaded sections (5) or similar positive protrusions, the neck portion (4) has a minimum wall thickness (w) that is smaller by at least 20% than a mean wall thickness in the region of the preform body (2).

12. Preform according to one of the preceding claims, wherein the neck portion (4) in the region of the threaded section (5) or similar positive protrusions, in particular on the threaded base, has a minimum wall thickness that is smaller than 1.34 mm.

13. Plastic container that is manufactured in a two-stage stretch blow-molding process from a preform (1) according to one of Claims 1-12.